Can I use SPLIT to obtain small and large RNA-enriched fractions from already extracted RNA?

Yes, this is possible. To do this, add 400 µl IB and 150 µl AB to a small volume of your total RNA (e.g., up to 100 µl). Mix thoroughly and add 0.33x volume of isopropanol to this mixture, vortex for 10 seconds, and continue at SPLIT User Guide, 9.1. Column Loading of Large RNA (step 18).