In some cases, yes. Twilio can add SMS capabilities to your existing Voice-capale US (+1) Toll-Free phone number, without porting your phone number into Twilio. This is called Hosted SMS.
With Hosted SMS, Twilio's Messages API can be used to send and receive SMS messages on your Toll-Free number, while the Voice portion continues to run on your existing phone system. This feature is currently in Developer Preview.
If you are interested in this feature, you can learn more about eligibility and how to get started in the Hosted SMS docs.