It really depends on what that different value is...
If you want to insert some known constant, you could use:
INSERT INTO tblname(f1,f2,f3)
SELECT f1,"foo",f3 FROM othertbl
If you want to insert the value at f2 but double, you could use:
INSERT INTO tblname(f1,f2,f3)
SELECT f1,f2*2,f3 FROM othertbl
If you want to insert the sum of f1 and f3, you could use:
INSERT INTO tblname(f1,f2,f3)
SELECT f1,f1+f3,f3 FROM othertbl
But I know those are probably the wrong functions to do the math, so I'll have to look up the right way to make my examples work, but the idea should be clear.