marcinq - I have done exactly what you are doing. By far the easiest way to do it is by modifying the bigip.conf file directly. That is a lot easier than adding them in one by one. Add one, look how it is in the config file, then repeat 253 times, using a spreadsheet or something to count up through the IP addresses. (I used Excel and exported it to .txt) Edit the bigip.conf file as necessary, and then reload the bigip.conf file back to the LTM, and run a "b load" command for 10.x at the CLI or "tmsh load sys config partitions all" command for 11.x... Hope this helps.
Nimbostratus