No, you can't. I can't tell what OS you are using, but if it's OSX or Vista, then the operating system is using the rest.
In either case, get some more physical RAM in your machine and ArchiCAD will start using it if it needs it.
If ArchiCAD is using 1G and there is actually some RAM free at the time(say, anything over 100M), then it's because ArchiCAD doesn't need any more memory, so it's not using any more.
Josh Osborne - Central Innovation
HP Zbook Studio G4 - Windows 10 Pro, Intel i7 7820HQ, 32Gb RAM, Quadro M1200