Yes, we can do that. You'd talk to Customer Service first (to add the HSI service to your account), then transfer to Internet tech support. We will need the Manufacturer, Model and HFC MAC ID from that modem to allow it to be authorized.
Most current modem models are supported - some older modems (Motorola 41XX) and DOCSIS 1.0 modems are not permitted. We currently use Motorola SB5101/5120 modems.
Be aware that the signal level at the coax outlet connecting to the modem must be within an acceptable range. If modem does not connect, disconnects frequently or exhibits performance problems, there may be a signal issue. Call Technical Support and the modem levels can be read from here. Correction of levels will require a technician appointment.