Troops of different levels will be converted to the same levels of initiator based on basic Health. If your race is different from initiator, conversion will be based on highest Health.

For example: Lv.1 Swordsmen with 1400 basic Health & Lv.2 Swordsmen with 2800 Health

If the Swordsmen of initiator are Lv.2, and your donated Swordsmen are Lv.1, these donated Swordsmen will be converted to 500 Lv.2 Swordsmen. (1000*1400/2800=500)