Yes, and as B-string said. However, there a few important things to note before trying to do this on your own:
1. Markbass considers the jumper changes to be something only an authorized Markbass service center is authorized to do, and not a user-serviceble action. So though it is possible and easy for you to do it yourself, proceed at your own risk as they may not repair the amp under warranty if you make a mistake and damage it.
2. It is still possible to use the effects return input for the purpose you described with the jumper still set to parallel mode. What parallel mode simply does is mixes the signal from the LMIII preamp with the signal that's plugged into the effects return - think of it like a two channel mixer. The only possible consequence of doing it this way is a drop more noise than the series mode, which is already negligible, but can be even further reduced by turning the LMIII input gain knob down all the way when you use this setup. The advantage of doing it this way is that you don't have to change anything inside the amp, and you still have the LMIII preamp available with its own gain knob for a second instrument or whatever.
3. The effects return is definitely pre EQ and NOT post EQ (including the VLE and VPF), whether set in parallel or series mode. So it is not simply a power amp input since the EQ follows it. However, with the 2 filters down all the way and the 4 EQ knobs set to 12:00 the EQ is virtually razor flat in frequency response and no "voicing" will be applied to the signal you plug into the effects return. This is actually a good thing because you still have the option to apply finishing touches with the LMIII EQ if you ever need to.