Sorry about the delay in getting back to you! I, uh, wound up sleeping for eighteen hours, thus missing most of Tuesday...

soc_puppet is correct: you and others can post to the community on Dreamwidth without any worries, even if the import hasn't finished yet. Any entries or comments that get posted to the community on LiveJournal after the point the importer gets your data from LJ -- not, I'm pretty sure, the point at which you schedule the import, but the point at which it reaches the top of the queue and starts processing -- won't be included in the import, but if there is anything that gets posted late, you can do another import next week (or whenever) to catch up with the stragglers.