Without a lot more information it is difficult to give you an accurate answer.
One of the things to be know is type of aircraft. If it is a part 23 aircraft the answer is yes and no. If you just remove the seats and load it up, it is not legal. You need to make a notation somewhere in the aircraft paperwork on the removal of the seats and the change in the weight and balance. The cargo needs to be tied down in an approved manner. And the crew needs to have an usable emergency exit. I have seen a King Air, where the crew crawled over the cargo and the loaders then threw more in and closed the door. That is not legal.
For part 25 aircraft it is much more complex. You have to comply with Part 25 which requires two (2) usable emergency exits. So care has to be given to the loading of the cargo. All the operators I have worked for required a maintenance signoff to change the configuration with a notation of the weight and balance change. They also required the installation of the cargo restraint system and a sign off. Then there is the requirements to meet floor, zone and section weight limits. Yes it can be done but it is not a quick spur of the moment item for Part 25.
The simple answer is that it can be done legally, but you have to jump through all the hoops.