When you studied Modals with regular verbs, you learned all the modals, and you should be able to do the following with ease:
Ons bak brood. (Add KAN/PAST)
Sue lees die boek. (Add WIL/FUTURE) etc.
So, what you already know is to add the modal to present tense sentences and to send the verb to the end. Then you know, to make that same sentence future, you would put the SAL in the second place, and send the modal to join the verb. And you would know that to make that same sentence past tense, the modal would stay in place, but take the past mask. KAN > KON.
Since modals take the past tense marking, there is no need for the GE- prefix, so regular and prefixed verbs and inseparable compound verbs are all treated the same.
2 - The Modals
KAN (can); SAL KAN (will be able to); KON (was able to)
WIL (want to); SAL WIL (will want to); WOU (wanted to)
MAG (is allowed to); SAL MAG (will be allowed to); MOG (was allowed to)
MOET (has to); SAL MOET (will have to); MOES (had to)
Note: Remember, the TO, like 'I will have TO sing'. This TO does not get translated. Because modals are so frequently used, memorize these words, and automate their use in all three tenses.