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.