I am new to all-grain brewing. What would adding a 250g of malt extract have on an all-grain brew?
Does "Full brew" mean that you will be adding the extract to a fermented beer, or are you just referring to an all-grain brew and adding the extract to the kettle?
I assume you can add extract to a fermented beer, but that seems strange. If you really want to, make sure that you sanitize it first.
If you are adding it to your kettle during boil, then it is nothing strange. This technique is often used to reach gravity if you are a few points short, or if you are brewing a beer that has an OG that is not obtainable from your mashtun. Lots of barley wines require a few pounds of extract.