There’s a long answer to this, since the short answer is somewhere on a continuum from No to Yes.
PBXact servers are considered commercial installations, so making drastic changes to them can affect your warranty and support status. If you are under warranty and don’t want to jeopardize your support, I’d stick with supported hardware and software.
Having said that, if you are using SIP-enabled Cisco phones and have installed Commercial EPM, then you should be able to get CIsco and Yealink phones working eventually. I always recommend against Cisco phones in SIP mode unless they come natively that way (some newer and some consumer models. for example). The phones that come as SCCP phones natively are a huge pain to convert and use, and are generally considered to be amongst the hardest phones to get working with FreePBX of any flavor.
If your Cisco phones are Skinny phones (7904, 7912, 7940, 7960, 7970, etc.) and you do not want to flash them to SIP, you can add the Chan-SCCP-B channel driver and SCCP Manager to your configuration to support these phone in SCCP mode in your LAN. Note that this may require you to install some additional support libraries to support the new software. Interesting new success with Chan-SCCP-B and SCCP-Manager (both from GitHub) is my post on setting this up without patching Asterisk.
If, instead of using PBXact (the appliance version of FreePBX), you are using the FreePBX Distro install, the above is the method I’d recommend for native Skinny phones.
So, bottom line: maybe. 