I have a CHR for VPN... this CHR has access to IPV4 & IPV6... If I connect via l2tp, for example, to the CHR via IPV4, it is somehow possible to access the CHR's IPv6 network by masking or something like that ?

I assume you've got a DHCPv4 server on the CHR end of that tunnel to assign an IP to the clients connecting via L2TP. If so, why not set up a parallel DHCPv6 server so the client gets both types of IPs?

There are a bunch of different IPv6 transition mechanisms, but I don't know if RouterOS exposes what the underlying Linux kernel offers in any way. These days, it's usually better to go dual-stack, as with my DHCP suggestion.