you can i beleive, but its fiddly.
you would have to first get the current waypoint that the group is on using getWPPos believe.
then change its condition with [grp, 2] setWaypointStatements ["true", ""], u'd have to make a variable for the waypoint number.
then setWaypointType to SAD (search and destroy), but also setWaypointPosition to the enemy forces (or current position), if on current position they should attack the enemy forces straigh away, but if you can set a search and destroy waypoint on the enemy by having a trigger that follows the patrol, gaining a list of all enemies, then (im not entirely sure how this bit works) using _x, select 0 (should select the first unit in the enemy group), get its position, and plonk the waypoint on it.
then, when ready to continue the patrol, meet the condition for that waypoint, and hopefull they should continue normally.
you'd have to reset all the bahviours of the group ofcourse.
hope this helps,
surdus