I'd say it is possible to do so. Although you have to create an individual importer by yourself. I've created one for another CMS a few years ago, had some issues but it worked. The way to do it would be roughly: [ulist=disc]have a look at the database structure of SMF and PHPF create some content in PHPF to identify the necessary fields and the corresponding ones in SMF read the database of SMF and convert the database fields in the one PHPF uses[/ulist] For example:
The table "SMF_topics" contains the threads of your board. The corresponding table within PHPF would be "PHPF_threads". Now here you have on both sites some information that could be easily transfered:
But now comes the tricky part: you need to get the subject of the thread since this information ist stored in PHPF within the table "PHPF_threads" but in SMF in the table "SMF_messages". Now I don't have SMF installed to verify but I assume that
SMF_topics.id_first_msg = SMF_messages.id_msg
therefore
SMF_messages.subject of SMF_topics.id_first_msg = SMF_messages.id_msg = PHPF_threads.thread_subject
Now there are severals tables which don't have a counterpart in PHPF, such as all log_*-tables. AND: importing the user-tables could be a pain in the ass since, if I've read Sbs-Auth.php correctly, SMF uses SHA1 with the password, PHPF still doesn't have a selection what kind of password protection you prefer. It is until now hard coded with SHA256. So you have either tell every single user that by changing the CMS they have to make a new password by themselves OR you change the encryption of PHPF by yourself to use SHA1 instead of SHA256 (although I don't recommend it).
Enjoy.
Edit: just found by Google that there are several importer-scripts available for SMF into other CMS/forums. Might be a good idea to have a look at them to get to know how to proceed for PHPF. Also have a look at e107, they come with an importer mentioned ony for user import of SMF buth the code also imports forums, threads and posts. And maybe #1434 of the Roadmap will be developed as well (among the other conversions scripts).