Sorry, not possible. Breach face won't be the correct size, Magazine and mag well, frame would need to be longer for the cartridge size. I am sure there are more reasons I don't know about. Maybe someone with a SW99 in .45 could help out with the differences (if they make one?).
According to what I read here it looks like there may be a couple of vendors looking into this....could be the 45GAP round could be more of a fit than a ACP as I would agree the fit could be problematic...
Max. Case Length
Case Size Comparison
Max. Cartridge Length
0.138”/3.51mm longer than the .45 GAP
0.09”/2.29mm longer than the .45 GAP
0.138”/3.51mm shorter than the .45 ACP
0.006”/0.15mm shorter than the .45 GAP
"Calling the 45GAP a “short .45ACP” is a misnomer since the ACP and GAP cases are quite different, both inside and outside. The GAP has an entirely new design. Unlike the .45ACP, the .45GAP case rim is slightly rebated (smaller diameter) so the case can be easily extracted/ejected in a 9mm size pistol. The mild rebated rim adds reliability for using the fat 45 case in a .40 sized pistol frame."