No need to share BTS.  Here is the solution:
Convert a small amount of your open.BTC to BTS to cover trading fees.  In the Bitshares wallet app, go to Withdraw and scroll down to INTERNAL CONVERSION.  Set the "from" currency to open.BTC and the target to BTS, then set the amounts.  Note that the "Insufficient funds" message highlighted in red on the right side immediately disappears.  Submit the request, then go to Dashboard to check your assets.  You should see a non-zero quantiy (what you requested) for BTS.  Now you can successfully submit trades to buy BTS with your open.BTC.