@Cire3 The short answer is that it’s possible, but it depends on how nextboot.xyz handles dhcp information.
The simples form is to add this to the fog ipxe menu builder parameter block.
chain tftp://192.168.1.12/nextboot.xyz || goto Menu
If nextboot.xyz uses dhcp information (which will point to the fog server unless we alter it.
set newserver:ipv4 192.168.1.12
set newbootfile nextboot.xyz
set net0.dhcp/next-server ${newserver}
set net0.dhcp/filename ${newbootfile}
set proxydhcp/filename ${newbootfile}
chain tftp://${newserver}/${newbootfile} || goto Menu