In brief yes it is, and I do this in my own Symfony2 project. I combine both SF-SE and SF-CMF bundles.
In fact, with Symfony2 it's very simple (this is just a matter of choosing the most suitable Bundles; SF is a very decoupled framework, which is why I don't plan to migrate to any other solution for the moment), but I'd like to share some of my experience with doing that. Actually the one most important question to think through to make a decision about how to combine both "worlds" is this:
Composer.
After some inquries I found out that since Symfony CMF is (in a way) based on Symfony SE, and not vice versa, it's better to start with the latter, as it contains the most core features (though I did it also in the opposite way, rather not recommended). So just take a SF-SE's composer.json, take a look at bundles from there you need, and then take a look at differences within SF-CMF's composer.json. You should end up with the most suitable set of bundles.
The basic features from these bundles to look-up for are:
- MODEL - Doctrine ORM, PHPCR-ODM, or both - if still not sure, don't hesitate to ask a comment, I'll share here my experience furthere.
- ROUTING - the primary question here is how flexible routing do you actually need? If not sure, I'd go with standard SF Router, and then possibly replace it e.g. when still on a dev stage.
- OUT-OF-THE-BOX CMS FUNCTIONALITY - bundles such as CreateBundle, MenuBundle or MediaBundle may help you building surprisingly fast, but not quite flexible soltions. In general, I ended up without using most of them, and if using, then I mainly take some Interfaces I do implement in my own Bundles (to ensure future compatibility with possible other bundles to be potentially used).
Besides of these above, I created a number of Bridge Design Pattern and Provider Design Pattern solutions to make some bundles working together, to adjust their functionalities, or simply to decouple things.