is there a opportunity to set a flag for specific files like the arc flag (-fno-objc-arc) in the compiler settings to use the 32bit lib instead of the 64bit lib?
The thing is, that I use a class with some functions they doesn't work in 64bit.
No, there is no source file-specific flag for specifying the bit architecture like you find with -fno-objc-arc.
This is because you cannot have a single program compiled partially for 32bit and partially for 64bit architectures, so you can't enable or disable this on a per source file basis like you could with -fno-objc-arc.