It is definitely possible to create a library (.lbr) from schematics (.brd) file using EAGLE Layout Editor. However, we couldn't find any relevant information if the opposite conversion works as well, but it probably does. You can most likely use symbols from a (.lbr) library when making a new schematics (.brd).
Convert EAGLE Library file to EAGLE Circuit Board file.
We found one software available in the database for this conversion.
Microsoft Windows software - convert lbr to brd on Windows
High quality PCB design software
Convert from lbr file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.