Yes. Any synths that are addons, e.g. SVOX Pico, can be copied into your
portable version with any other addons. Addons you have to license, like
Vocalizer, can be licensed, e.g. if you do a copy on a portable USB key.
I don't know how something like SAPI 5 Eloquence would work with
licensing, it might not since it would need to be installed, but if you
installed it to a USB drive, maybe it would, although I don't know if a
Windows system you plugged the drive into would search the drive for a
SAPI synthesizer rather than its own installed files. The Windows 10
OneCore voices will work on any Windows 10 system, same with any SAPI 5
voices installed on Windows, depending on the version. So yes, you can
use other synthesizers in a portable copy of NVDA.