Your seltzer thing has a pour valve that seals tight yes? leave it empty but for a small bit of dry ice. Vent the mixed gas then charge with the cartridge and you have co2 on tap.

Or simply use dry ice. Take a new garden sprayer. The ones you pump. drop in dry ice and as it warms it will pressureize with CO2 and it has a handy wand built in for ease of use.