This depends on whether they are open, shielded or sealed. Open bearings are usually lubricated with oil which means they can easily be re-lubricated whereas shielded or sealed bearings are generally lubricated for life with grease.
Most shields cannot be removed and replaced so the bearing cannot be re-lubricated without specialist equipment. Rubber seals can be removed (carefully) for cleaning and relubrication but generally, shielded and sealed bearings are not designed to be re-lubricated by the user.
Some shielded instrument bearings may be oiled provided speeds and loads are low and rotation is intermittent. These bearings can be re-lubricated by putting one or two drops of oil into the gap between inner ring and shield and spinning the bearing to distribute the oil and remove surplus oil. The oil should be of the same type as already in the bearing as different types of oil won’t mix.