You can actually do this rather easily I believe:
Make a condition so when (player) kills a unit 1 point is added to kill score.
This can be done with the KILL condition and the SET SCORE action.
(You may only need the bottom trigger, I'm a little rusty since using the Warcraft 3 map editor for so long)
Next trigger:
For the condition use the SCORE condition:
Make it so when (player's) kills score is 25
Then for the action use the create unit however you need
Then use the SET SCORE action to set the kills score back to 0
PRESERVE TRIGGER
I'm about 99% that should work.
Make a condition so when (player) kills a unit 1 point is added to kill score.
This can be done with the KILL condition and the SET SCORE action.
(You may only need the bottom trigger, I'm a little rusty since using the Warcraft 3 map editor for so long)
Next trigger:
For the condition use the SCORE condition:
Make it so when (player's) kills score is 25
Then for the action use the create unit however you need
Then use the SET SCORE action to set the kills score back to 0
PRESERVE TRIGGER
I'm about 99% that should work.