You may certainly install the PTFs via CSM, even when the product has been installed outside of CA CSM.
Since you installed the product outside of CSM, you will first need to migrate your CSI into CSM from the "SMP/e Environment" tab.
To install the maintenance, you can go to the Products tab. On the left hand side of the screen, locate the product and the release. Then, right click on the release number. Final step, please click "Get Latest Maintenance".