I haven't seen any of the Rational or IBM products for years; I assume that they don't provide any standard metadata export mechanisms except for XMI. Relying on XMI for metadata interchange between tools is notoriously flaky, especially in the case of Rational Rose, an ancestor of Rational Software Architect.
The out-of-the-box method would be to import into PowerDesigner from your Rational XMI files, into PowerDesigner Object-oriented (OOM) models. If necessary, you could then generate a CDM, LDM or PDM from the OOM. It's likely that the end result may not be to your liking. If the Rational models match real-world artefacts, you might be better off reverse-engineering those artefacts into PowerDesigner instead.
If your target is the CDM, LDM or PDM in PowerDesigner, you could use a more direct route, using the Reischmann Toolbus (Reischmann Informatik GmbH - TOOLBUS Interfaces) - they have a number of inexpensive bridges between PowerDesigner and IBM Infosphere Data Architect, and also between PowerDesigner and IBM Software Architect. If none of the bridges does exactly what you want, they have a reputation for being flexible in how they convert metadata.
There are also other bridges from Meta Integration (Meta Integration® Model Bridge (MIMB) - SUPPORTED TOOLS), which are often built in to modelling tools, but not PowerDesigner.
The Reischmann bridges are dedicated to individual tools, whereas the Meta Integration bridges are generic, importing into or out of a common meta-model.