Should be fine for a standard C172, just not some of the more exotic variations - you can't add variable pitch and retractable gear.
There's no legal issue with what you want to do, and if you're already an experienced glider pilot, then I think that you're likely to be the right person.
I'd suggest going and talking to the chief tug pilot and explaining the aspiration - given that you've not started the NPPL yet, there's then plenty of time to deal with any particular requirements he or she may wish to make.
A C172 is a nosegear aeroplane. Some clubs estimate five hours of training for a tailwheel conversion - which would allow you for example to fly a Chipmunk; however, if you have a lot of gliding hours, then it really shouldn't take more than a couple. Better still, just look for somewhere that you can do your NPPL in a taildragger, saving having to do the conversion.
G