You can, but better if you don't. SquashLevels is a results based system that assesses how well you're playing based on your results and the level of your opponents. If you play a friendly then it's likely you won't be playing a truly competitive match and your level will get dinged. We do allow a range for the better player to play in but if you drop games to someone half your level then you're going to be going down... and, equally bad, your opponents will be going artificially up.
We will add a future feature to allow you to enter your results but exclude them from the level calculations(that's really intended for dealing with injuries) but that's not available yet. Watch this space on that one.