You can probably use Set Column Value(SCV) feature in Origin. See the picture below, put the intercept value in C1, open SCV dialog of col(D), put the script in.
range r1 = col(C);
fitLR iy:=(1,2) fixint:=1 intercept:=r1[1] oy:=col(D);
Then you can change the intercept value and the fitting result gets updated.
James