I've noticed a few people on the SSIS forum lately asking about expressions on the Foreach Loop. They are aware that its possible to set expressions on properties of the Foreach container but are flummoxed when it comes to setting expressions on the properties of the enumerators themselves. That's understandable because the properties of the chosen enumerator are not available from the "Property Expressions Editor" on the "Expressions" tab of the "Foreach Loop Editor" (which is where they usually appear). You can see that here:
All is not lost however. There's a seperate route into expressions on enumerator properties via the "Collection" tab of the "Foreach Loop Editor". I've highlighted that on the next screenshot that shows properties being set on the Foreach File Enumerator.
Note the "Expressions" section on the "Collection" tab. That's the bit you're interested in.
Dead easy when you know where to find it! I must admit it took me a while until I discovered it.