How to configure switch stacking on Dell Z9100-ON switches

This can be done, see the page 15 of the Dell white paper on the subject here: http://www.dell.com/downloads/global/products/pwcnt/en/pwcnt_stacking_switches.pdf

Specifically it says:

Merging Two Operational Stacks

The recommended procedure for merging two operational stacks is as follows:

1. Always power off all units in one stack before connecting to another stack.
2. Add the units as a group by unplugging one stacking cable in the operational stack and physically connecting all unpowered units.
3. Completely cable the stacking connections, making sure the redundant link is also in place.

Two operational stacks can also be merged by reconnecting stack cables without powering down units in either stack. Connecting a powered up standalone unit to an existing stack leads to same behavior as when merging two operational stacks. In such cases, Manager reelection is done and the Manager with the higher MAC address wins the election. The losing stack manager resets itself and all its member units. After the reset, all the losing stack members join the winning stack to form a single stack. The winning stack remains functional through the merge process. If the stack merge is performed in this way, then it is strongly recommended that the user set the admin management preference of the desired winner stack manager to a higher value than the stack manager that should lose the election.

Also, in your above proposed cabling you are not completing a loop with your stacking cables. You should always aim to do this, even though it is not a requirement. See page 7 of the Dell m6220 stacking use case: http://i.dell.com/sites/content/business/solutions/whitepapers/zh/Documents/powerconnect-m6220-switch_cn.pdf

Cabling:

M6220 switches should be stacked together by connecting a stacking cable from stack port xg2 of one switch to stack port xg1 of the next sw itch. The stacking cable is included with the stacking module. Stack port xg2 of the fi nal switch in the stack should be connected to stack port xg1 of the first switch, completing the stack loop. A loop topology is not strictly required for a stack but it is recommended, as it is the most robust stacking topology and has seen the most time in production use.

Went trough the Dell documents and it doesn't explicitly say anything either way about stacking different platforms together. However the complete lack of documentation regarding stacking different devices together sends a pretty clear message that they are only intended to stack with devices of the same model/line.

More generally, most stacking solutions require that the the stacking technologies and the software versions be the same for switches to participate in a stack. I am not sure if you can run the same version of code for the two platforms, but clearly the stacking technologies are different. While there appear to be other differences in the stacking technologies as well, I will simply state that there is no way to connect the 10Gig HDMI stacking ports to the 1Gig RJ45 stacking ports.

You can keep the older 3448P switch and use it, but it will have to be configured and managed separately. The "stacking" ports on the 3448P should be usable 1Gig ports and can t hen be used to connect to your new switches as noted in the Dell documentation for the 34xx switches ("two Copper ports which can be used to forward traffic in a stand-alone device, or as stacking ports when the device is stacked"). Exact configuration of the devices would be entirely dependent on your environment (do you need tagged/trunk links, etc).

Btw, I can't find documentation of stacking for the 5448 (aside from the occasional use of the word "stack" and most of those had to do with dual stack IPv4/IPv6), so I am guessing that you meant to type 5548 which matches your description.