Hi,
The error you received indicates that at that particular moment in time the Feed to Post add-on was active, but the core WP RSS Aggregator plugin was not. The core plugin must always be active when add-ons are active. Under normal circumstances, it should be impossible for an administrator to activate one of our add-ons without having the core plugin active; however, it is possible that this happens as a result of some error internal to WordPress, such as if the core plugin fails to update for some reason. So, to answer your question, it should not be necessary to de-activate add-ons to update the core plugin; however, in your case it may be necessary, if you continue receiving such errors.
We will look further into this, and if we find a problem that we can fix, the fix will be included into one of the future releases. Thank you for reporting this. If you continue having this problem, or other problems related to add-ons, please get in touch with us via the Premium support service.