Yes you can use synthetic blend oil with regular oil and it will do no harm to your engine.
If you have some synthetic blend oil in your engine and you want to add some regular oil to top off the engine oil then you can do so without any issues.
The same goes for regular oil already in your engine and adding some synthetic oil to the regular oil.
The two types of oil synthetic engine oil and regular engine oil will blend together just fine and your engine will not be harmed.
I've mixed synthetic engine oil with regular oil several times before and still do sometimes when I just need to top off the engine oil when I have synthetic oil in it.
The regular oil is cheaper than the synthetic so I top the engine oil off with regular oil when I have synthetic oil in the engine.
It's never been an issue.