Hey,
For as far as I know you can't add a driver for a printer in an appstack (not 100% sure though because the spooler is being restarted after appstacks are attached) if you attach the device locally and you would need to passthough the device on your ThinClient even if it would be possible. Also, your TC would need to support this.
What we did is we connected the Dymo printers to a Dymo printserver using this thing (yes, I'm Dutch DYMO LabelWriter Print Server - Printershop.nl ) , created a print queue for it on the print server and installed it into the VDI using a script to install the printer. Manually installing works as well. The thing is that you don't need to worry to much about drivers because it is network connected. And now you are able to share it with multiple users.
We then created an Appstack that ONLY holds the application for the Dymo Labelwriter (not the driver, please do not select it during install) and gave permissions to people that have permissions to a Dymo Labelprinter.
The thing is that the Dymo software actually looks for a Dymo Labelwriter with it's corresponding driver when starting up. If it can't find a Dymo printer it simply wont work.