I take it you are importing the ground as a static entity. Make sure when you define your FPE that your collision value is set to 'full polygon collision'. I cannot remember off hand what the value is, but I know that the WW2 buildings use this collision mode so check our those FPE and copy that collision value and you will be able to place and walk over your new ground tile. You can also do this as a segment, but that is QUITE anothing story
"We are the knights who say...eki eki eki fatang loopzoing, zanziga....ni"