I recently bought a very cheap (due to it having no remote control and centre and rear surround speakers) Sony BDV-E370 HT system.
I managed to get a new remote through Ebay and while it's usable with the sub-woofer and FL and FR speakers, or through the TV speakers over HDMI, I was wondering if could add the centre and rear surround speakers from a redundant Technics Pro-Logic hifi?
The Sony set up has 3 Ohm speakers and the Technics speakers are 4 Ohm
Yes, you can add a 4 ohm speaker. As long as the ohm rating is equal or higher than what the system is rated at, you are OK. Unfortunately, a higher Ohm rating on the speaker will mean more resistance and less rated power through the speaker. It will work without damaging anything, though.