Subwoofers take a fair amount of power to get the best from them, your amplifier is unlikely to be able to deliver enough power to your main speakers and a passive subwoofer, to fill a 5m x 5m room with good quality, unclipped audio.
But, in theory, it could work as you describe, in practice, it is unlikely to work very well.
If you use a passive subwoofer, it would be usual to use a separate mono power amplifier to power it, which you could feed from the sub out on your amp. But, it is not really worth the expense and the hassle, so getting an active subwoofer, with a plate amp, controls and the speaker drivers all in one box, is much more convenient.