Let me say the most important thing first. English has a definite progressive tense and we use it proudly. Afrikaans does not. We are comfortable of saying things without the this tense and let the context tell you is is ongoing. If we have to emphasize the ongoing nature, we add words to the sentence to make that known.
HEDE: I am working on a plan. > Ek werk aan 'n plan.
TOEKOMS: She will be helping him. > Sy sal hom help.
VERLEDE: We were calling the cops. > Ons het die polisie gebel.
So, what then if you really want to emphasize in Afrikaans that you were in the process of something. Here are some solutions:
Jy moet wakker word! Jy is aan die slaap. (You have to wake up! You're fast asleep.)
Sy was aan die kook, toe ek tuis gekom het. (She was cooking when I got home.)
Was jy aan die slaap toe ek gebel het? (Were you sleeping when I called.)
"besig om te / besig wees om xyz te [verb]"
Ek was besig om te slaap toe jy gebel het. ("I was busy to sleep when you called.")
Ek sal jou nie kan help nie. Ek sal besig wees om te skryf. ("I will not be able to help you. I will be busy to write.")
Note: The OM TE is the Afrikaans infinitive and will be taught later on.