Installing exterior doors in pole building is not unlike installing doors in stick or conventional framing. The biggest difference, you will need posts to frame the door opening. Set your post the distance of the rough opening size for the door you will be installing.Once the posts are set, plumb them in both directions. Purlins or horizontal 2x lumber is normally installed on the exterior of the posts to attach siding. Cut these framing members to the inside of the posts, then cut vertical blocking to go in between the horizontal spacing (normally 24" o.c.) This will give you a solid surface to attach siding and the door from the exterior.
A typical exterior door installation- service or man door is installed by plumbing and leveling the door just as any other. One thing you need to be aware of is the jamb width of the door. Typical door jambs are set up for 2x4 or 2x6 wall construction. If you have a 6 inch post and add an additional 1-1/2" for blocking and framing on the outside of the post, your jamb width gets really wide. This can create a problem as the door won't open a complete 180 degrees, it will hit the posts and only open to 90 degrees. Planning ahead can eliminate any potential problems this can cause. Order a thicker jamb size or plan your door close to a corner or inside wall will solve this issue.
Rolling and slider doors and installed very differently, I would recommend what is called a "cannon ball track", this system utilizes a roller that is round in shape and is less prone to jumping the track. You will also need a header or solid framing member to attach the track system to. These doors will need to be slightly over-sized to "cover" the finished opening.
Post and beam buildings can be fun to build, great looking and long lasting structures. Difficulty of building is not any harder than typical framed structures. Depending on the size of beams and posts, additional equipment, like cranes or forklifts may need to be rented or purchased.