There is no AUTOMATIC way of doing this, but you can easily control the appearance of enemy (or friendly) aircraft by the creative use of Events and Flags.
For example:
Lets say the player is flying 100 nm to the target, and the enemy airbase is 50 miles from the target,
You can use an Area or Moving area event to trigger the takeoff of enemy aircraft when the ingressing PLAYER is ~60 nm from the target. This way, assuming you placed proper waypoints/speeds for the enemies, the enemy aircraft will arrive on scene at about the same time as the Player.
Similarly, you could have the enemy aircraft in an ORBIT near the target and use the flag to have them break orbit and head for the target WP.
With a fair understanding of the events and flags available in the mission builder, you can arrange ALMOST anything..but it takes some trial and error to get things just right..