I've found a way to replace your lethal grenade and tactical grenade with any weapon that you want. In the following image and video, I have replaced my lethal grenade with the R870 and I replaced my tactical grenade with the Vector K10. This can also be done with any other weapon. So, I have the MK48, SMAW, R870 and the Vector K10 all at the same time. You access the other weapons just like you do the primary and secondary, just scroll the mouse wheel.
This can also be done with perks, and I'm in the process of trying out scorestreaks.
I will make a trainer for this as soon as they update the game again around the 28th.