I don't understand what you are asking?

DKIM is simply a txt entry in a domain dns zone file.

if you could detail on this; I would be happy to help.
Usually DKIM are added as TXT record on email sending domain name. Few email relay services my use CNAME record; apart from TXT.

I would like to keep DKIM signature generating their key automatically, i do not want to fix one key

That wouldn't really help due to the fact that different servers around the world refresh their dns cache at different times, so you would be sending email to one server with DKIM 1 in place, and when you are sending email to the second server with your new DKIM 2 in place, it is still showing DKIM 1 in it's cache, so your deliverability would go down.

As SendEmails says there is no use of what you're trying to achieve.
I pressume you want to have multiple DKIMs because you have more than 1 ESP for the same domain?

DKIM are added as TXT DNS record, If you're trying to rotate DKIM's assign multiple subdomain/domains and rotate the domains