Can I use looping constructs with @Commands? Basically what I want to do is create a number of fields specified by the user.
Notes/Domino 6 supports looping in macro language using @For, @DoWhile, @While or @Transform. Read about them in Domino Designer help. You can use @Commands in the loop, but bear in mind that not all @Commands execute immediately when they are encountered in the macro -- some of them wait until the end. It's not clear to me from your question why you would need an @Command to create a field. What does that mean, exactly? So I'm not sure what command you would be trying to use there.
There's a lot you can do in R5 as well, just using the fact that most of the @Functions and operators will iterate through the values in a multi-value field. However, there's no way in R5 to call @Commands repeatedly based on the values in a list. LotusScript (or Java) would be needed for that type of application. You might find it easier to use LotusScript for something like this even in Notes 6.