Thou Shalt Not Pass
You can easily avoid the outposts without the need of obtaining a pass from the baron.
This task activates after reaching one of few outposts at Velen’s border (M2,9) where Redanian army has placed barricades and no one without a proper document is allowed to get through. You can bribe the person you talk to (10 crowns) or use the Axii sign (25 experience points) to learn how to obtain a pass. However, these actions aren’t mandatory. The document can be obtained in two ways. The first option is completing the Fake Papers (M2,6) sidequest in which you buy the pass from a shady merchant. Second option is reaching enough progress in Family Matters main quest. Baron Philip Strenger will give Geralt a Letter of Safe Conduct after he learns about the whereabouts of his family. After you obtain the pass or the letter, you must show the document to the guard and he will let you through (75 experience points).
If you don’t have the pass and you don’t want to wait until you obtain it, you can simply bypass each barricade by, for example, using the boat to swim to the other side.