In solving an op amp problem with two voltage sources, why can't we combine it?
Do a 'sanity check' on your proposal. Set Vin = 0. 

We know that Vout will be \$V_{OUT} = +V_{OFFSET}(1+\frac{R_F}{R})\$, which is not equal to what your equation yields: \$V_{OUT} = +V_{OFFSET}(\frac{R_F}{R})\$, so no you cannot. 

The gain from the non-inverting input (Voffset) is higher than the magnitude of the gain from Vin. 

If you want to subtract two voltages and have equal gain from each input what you do is add a voltage divider to the non-inverting input that is of the same ratio as the feedback network (R/Rf) and that reduces the overall gain from that input to Rf/R. 

However if that voltage is fixed, you can save two resistors by simply adjusting the value of Voffset lower by that factor.