Yeah. Read up on certificates. That's just having the SBC make a request and asking an authority to sign it. In your case, the authority is SMGR. The only thing that will change on SMGR is in it's CA logs, it'll show it signed a request and generated a certificate for something. It won't change anything about how SMGR functions.
When you import it in the SBC, you can assign it to a TLS profile which can then be applied to a signaling interface. If you're doing TLS before getting a cert signed, then you're default TLS profile in the SBC uses the out-of-the-box certificate.
So, if we had an SBC and wanted to SIP trunk to one another, we could have that SBC issue 2 CSRs for 2 TLS profiles - ViveksAura and KylesAura. Kyle's SMGR will sign one, Vivek's the other. The signaling interface from my SM towards that SBC would use the TLS profile with my SMGR signed cert, and you would use yours. Now, when the SBC connects to my SM, they'll exchange certs both issued by an authority they trust, and when it connects to yours, it'd use the certificate signed by Vivek's SMGR. The SBC would then act as a nice trusted box in between our networks and in both our DMZs. Make sense?