Magento – Creating a Customer Attribute set by Dropdown Populated from a table

I have a custom module that created the table Namespace_Module_Rep_Directory_Rep. In this table we have a set of locations with entity_id, name, email, phone, status.

I want to be able to assign each customer to a rep. I know how to make customer attributes, but I am unsure how to make one that is linked to this table. I want to be able to go to a customer, and for "Rep" select a rep from a dropdown menu populated with the names of the reps from Namespace_Module_Rep_Directory_Rep.

I have looked all over for tutorials or similar setups, but have yet to find anything really useful. If anyone can provide some insight on how to accomplish this or a site that may help me I'd appreciate it, thanks.

Update:

I have followed @Marius's answer and have run into a problem implementing it. When I go to edit a customer, I get the error:
"Source model "namespace/attribute_source_module" not found for attribute "module"

It seems to be unable to find the model with the options for the select, so it errors out. I followed the example here, which @Marius provided.

Namespace/Module/etc/config:

...
    <global>
        <models>
            <module>
                <class>Namespace_Module_Model</class>
            </module>
        </models>

...

Namespace/Module/Model/Attribute/Source/Module.php:

<?php
class Namespace_Module_Model_Attribute_Source_Module extends Mage_Eav_Model_Entity_Attribute_Source_Abstract{
    protected $_options = null;
    public function getAllOptions($withEmpty = false){
        if (is_null($this->_options)){
            $this->_options = array();

            $this->_options[] = array('label'=> $this->__('Option 1'), value=>1);
            $this->_options[] = array('label'=> $this->__('Option 2'), value=>3);
            $this->_options[] = array('label'=> $this->__('Option 3'), value=>3);
        }
        $options = $this->_options;
        if ($withEmpty) {
            array_unshift($options, array('value'=>'', 'label'=>''));
        }
        return $options;
    }
    public function getOptionText($value)
    {
        $options = $this->getAllOptions(false);

        foreach ($options as $item) {
            if ($item['value'] == $value) {
                return $item['label'];
            }
        }
        return false;
    }
}

Namespace/Module/sql/module_steup/install-0.1.0.php

<?php
$installer = $this;

$installer->startSetup();

$setup = new Mage_Eav_Model_Entity_Setup('core_setup');

$entityTypeId     = $setup->getEntityTypeId('customer');
$attributeSetId   = $setup->getDefaultAttributeSetId($entityTypeId);
$attributeGroupId = $setup->getDefaultAttributeGroupId($entityTypeId, $attributeSetId);

$installer->addAttribute("customer", "module",  array(
    "type"     => "int",
    "backend"  => "",
    "label"    => "Module",
    "input"    => "select",
    "source"   => "module/attribute_source_module",
    "visible"  => true,
    "required" => false,
    "default" => "",
    "frontend" => "",
    "unique"     => false,
    "note"       => "Module"

        ));

        $attribute   = Mage::getSingleton("eav/config")->getAttribute("customer", "module");


$setup->addAttributeToGroup(
    $entityTypeId,
    $attributeSetId,
    $attributeGroupId,
    'module',
    '999'  //sort_order
);

$used_in_forms=array();

$used_in_forms[]="adminhtml_customer";
//$used_in_forms[]="checkout_register";
//$used_in_forms[]="customer_account_create";
//$used_in_forms[]="customer_account_edit";
//$used_in_forms[]="adminhtml_checkout";
        $attribute->setData("used_in_forms", $used_in_forms)
                ->setData("is_used_for_customer_segment", true)
                ->setData("is_system", 0)
                ->setData("is_user_defined", 1)
                ->setData("is_visible", 1)
                ->setData("sort_order", 100)
                ;
        $attribute->save();



$installer->endSetup();