Fiserv appears to be able to use customer domains in the SMTP envelope, which would allow for SPF to relevant in a DMARC context.
There's data that Fiserv partially supports DMARC compliance via DKIM either for a very limited number of clients or for a very small part of its infrastructure. Contact Fiserv to understand more.
If you add 'include:fiserv.com' to your SPF, you'll authorize a list of servers that's too large and pulls in 4 additional DNS lookups. Instead, you could ask Fiserv to provide a specific list of IPs to authorize only the servers which you use.