You can expand your present door framing for very little cost or effort. The main thing is that the door frame you are going to widen can not have ceiling support beams on both sides. These are the beams in your home that hold and distribute the weight of your roof and or second floor.
Step 1 - Removing the Old Frame
The first thing you will want to do is remove the door framing and remember where your trim used to go, as every doorway is a little different. You will want to use your stud sensor to check either side of your door for the studs distance into the wall cavity itself. If the stud reads deep into the wall it is likely a support beam or floor joists, in which case you'll need to check the other side. You will also have to take into account the actual space you have available to you on either side of the door.
Step 2 - Removing the Support
Once you have found the side of the door that you can expand the inches you need, look on the inside of where the 2x4 is exposed on the top and bottom to locate the securing nails in the door framing. You can use your flat bar or nail puller to remove what can now be considered a temporary support beam. Try not to damage the beam as you can reuse this once the doorway is established. Now remove the other sides support, and the cross support beam that runs above the door frame.
Step 3 - Cutting the Inches
When you are widening your door framing you will need to actually cut chunks out of your existing wall, once the support beams have been removed. You will want to add an inch for your door bracing, dummies or shims, so you will have to compare the exact measurements for your particular door. Take your saws and cut out the extra wall to be even with where your door frame will come flush. Be careful not to over cut your door opening into your wall. You can do this by using a ruler and pencil to make a guide line for the cut.
Step 4 - Rebuilding the Frame
You then will remount a new top support beam into door framing and measure it out to the length of where your other support beam will be, in some cases you may have to cut the vertical beams as they extend up into the wall. Either way use the cross beam to attach to all possible ends securely.
Once you have the support beam secured into the door framing, you will then secure it with the vertical supports you removed in the beginning. Be sure they are secured at the bottom and top using your gravity level to check for angle. You want the outer support beams to be even with your hanging weight level to avoid installing your door on a tilt. Install your new door and then construct your new trim for the top of the door, and your done.