Will VSTO add-ins developed for Office 2007 work in Office 2010

No, unfortunately it's not possible. VSTO 2010 will not compile solutions designed for Office 2003. It is designed to work with with 2007/2010.

To create solutions for Office 2003 you'll need VSTO 2005 on VS2003 or VSTO 2005 SE / VSTO 2008 on VS2008 (in the case of the later, you can also develop for Office 2007).

Using VSTO, you'll probably need different add-ins (see the table of compatibility in the English Wikipedia article about VSTO).

If (1) you need to bring your solution rapidly to the market, and if (2) you have reasons to belief that your codebase will need frequent updates in the future, you may consider to use a third-party tool like Add-in Express, which lets you create what you need with less headaches.