Alright, first of all, make yourself a "Radioman" and make them buildable in the barracks. (Make a copy of engineers, and rename them to radioman, follow the Grand How-To and make sure to have only 1 member in the squad)
This section is in attrib->ebps->races->allies->buildings->barracks.rgd
Now, go to the ebps of any production building, I recommend allied barracks and copy the spawner_ext from there (right click copy)
Go to attrib->ebps->races->"race"->soldiers->"radioman".rgd
and paste it into the GameData of your radioman (right click on GameData, choose paste into)
making sure it adds a spawner_ext to the list.
now select the spawner_ext and set spawn_at_map_entry_point to true.
Now delete engineer_ext to remove any construction menus (right click, delete), and remove any abilities from the abilities_ext (dont delete "ability01" etc, simply click the abilities list and remove any entries from the right hand side, leaving them blank.)
now simply add whatever squads you want to be called in to the squad list of the spawner_ext, and make sure the unit you want to call in's ui_group is between 5 and 12, and there are no repititions (again, refer to the Grand-How-To to do this, exactly the same method as adding your radioman to the barracks).
go ingame, build your radioman and start building those reinforcements!