Yes. Many organizations that are already using ISO have chosen to adopt CMMI to leverage their processes beyond compliance to continuous improvement.
In particular, the synergy between ISO 9001 and CMMI is high. But the emphasis in CMMI is on assuring institutionalization – across multiple projects – that represent the organization being appraised. As a result, we sometimes find that a "Maturity Level 3" organization easily passes its ISO auditor's look, but that ISO organizations are sometimes not of "higher maturity."
Since the two frameworks venture into slightly different areas of coverage, a single "system" to cover both would necessarily be a bit larger than that required by either one. It is important to emphasize that it is the continuing use of the model in development projects that is most important for CMMI. Thus, a "QMS system" may establish a wonderful infrastructure, but if the development teams are not continuously improving their processes, the result may not be very well rated in CMMI.
Several lead appraisers have built their data gathering tools so that they can use artifacts that satisfy one need and also satisfy the other.
In general, CMMI is more detailed than ISO 9001, and covers areas of the business that are often ignored by 9001 implementations, so in that sense you could consider it stricter. On the other hand, the areas that CMMI covers are generally well-related to the coverage of ISO 12207, which helps to "fill in the gaps" for ISO 9001.
The CMMI Institute is continuing to explore how CMMI and ISO can benefit each other. Although the following resources are based on the prior version of CMMI, the ISO-to-CMMI version 1.2 mapping documents (ISO 9000; ISO 12207; ISO 15288) attached below may provide a sense of how the frameworks overlap.