VPN behind existent routers

Hi,

I hope you can help me, I have 3 diferent locations, and I want comunicate thos 3 by tunel vpn with Mikrotik router on each site, the problem is that those three locations already have a router, I think those are fortigate routers, is it posible to make this comunication by vpn but with the mikrotik routers behind this router, I mean the public Ip will not be on the WAN port of the Mikrotik, it will be on the fortigate.

I would appretiate your help,
Thanks,
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