Thanks Vadim!
Using BAS() is a great idea. Im ashamed to not have come up with it myself. I hope it sums up the base memebers of 450 into 50 though 
.
Also, apparently this code works
*XDIM_MEMBERSET C_ACCOUNT = 450
[C_ACCOUNT].[#50] =[C_ACCOUNT].[450]
once I specify the scope for each dimension, but this version has some other drawbacks in future development of the functionality I need concerning variable usage in the
[C_ACCOUNT].[#50] =[C_ACCOUNT].[450]
line and concatenation with strings.