The answer will depend on the exact model of 300PL you have, but there’s a good chance it will be possible to use a 5.25” drive with your computer.
Many 300PL models, such as type 6592, type 6565, and your type 6862, explicitly support 5.25” drives, and support connecting two drives at once; so with one of those, you should be able to connect the drive, configure the BIOS and go. The 6862 at least is supplied with a very short cable with only two connectors, so you will need to replace the existing floppy cable with one appropriate for 5.25” drives, which normally use an edge connector. Compatible cables looks like the one shown here, with five connectors and a twist.
Given the physical layout of the 6862, you might have trouble routing the floppy cable if you try to connect both the 3.5” drive and the 5.25” drive at once. You will at least need a rather long cable, and in particular one with a decent length in between both sets of drive connectors (where the twist in the cable is found). You’ll probably find it much easier to only connect one drive at a time.
Other types might or might not directly support 5.25” drives; for example, type 6562’s manual claims it doesn’t. However if the BIOS doesn’t support the drive, you may be able to configure Windows to use it anyway, and if that fails, you can buy a cheap ISA floppy controller and plug it into one of the ISA slots.