I will try to emulate your setting.
Yes. Just add TXT record to the template with __base__ at the beginning:
v=DMARC1; p=none; ruf=mailto:d@ruf.agari.com; rua=mailto:d@rua.agari.com; fo=1
Rebuild the DNS for domain.
Then remove the entry in DNS template and add correct TXT record with _dmarc:
v=DMARC1; p=none; ruf=mailto:d@ruf.agari.com; rua=mailto:d@rua.agari.com; fo=1
Then rebuild again.
You will have two records then:
one with __base__ and one with _dmarc which is incorrect.