Sponsorships would help, but most sponsors are local, and Des Moines has probably largely tapped into that. TV deals? For NASL? At best they break even on their TV contract now. They may very well be paying to have things broadcast. Marketing takes money. Where will that come from? Marquee players take money. Where will that come from?
You are aware that NASL and it's teams are mostly paying to have their game on tv? Much like MLS was doing not to long ago. So there really is no tv money to be had.
Maybe the NASL should try building a ladder (3rd div NASL?) before trying to coax a traditionally dominant PDL club into a potentially ill advised and comparatively costly elevator ride to a 2nd division NASL just so they can become that unstable league's new doormat.
I think the NASL needs a footstool first. Ladders can come later. (Not to mention that, in the scenario described, Des Moines would have to ditch the USL to join the NASL, which has not been the case lately and is not terribly likely.)