Adding custom placeholders in DNS template

Dear all,
I have a multi-server ISPConfig setup (master-slave).
I would like to edit the default template so that the MX record is associated with the hostname of the slave server where the DNS zone is created.
I tried searching for other DNS placeholders, but the ones available seem to be only:
{DOMAIN}, {IP}, {IPV6}, {NS1}, {NS2}, {EMAIL}.

I would kindly like to ask if there are any other placeholders, such as {SLAVESERVER-1}, {SLAVESERVER-2}, ..., so that they can be used for assigning the MX record.
Example:
{SLAVESERVER-1} = server1.example.com
{SLAVESERVER-2} = server2.example.com
...
--> such as, in the DNS template:
--> MX|{DOMAIN}.|{SLAVESERVER-1}|10|3600

If this is not possible, I would like to know (with some detail/instructions) if I could create custom placeholders to achieve the same purpose:
MX|{DOMAIN}.|{SLAVESERVER-1}|10|3600

I would also like to know if it is similarly possible to add DKIM and DMARC for the domain in question on the slave server, via the DNS template as well.

Looking forward to your kind feedback, I thank you in advance.