Translation:Ik weet niet of ze van soep houden.
"Als" is only used to describe a condition: If X, then Y.
If in the sense of whether, like in this sentence, is always translated as "of".
"Of ze van soep houden" is a subordinate clause, so the verb goes at the end.
Of is also used to join coordinating conjunctions, I'm assuming there is a difference in the context of it meaning "if" (whether) rather than "or" (an alternative) ?
So would it be correct to say "Ik weet niet of zij de soep lekker vinden"?