In older versions of SPSS, like SPSS 11.5, a linear regression model can be exported to PMML by going through the following sequence of menus: Analyze -> Regression -> Linear... -> Save... You will find yourself in box "Linear Regression: Save". Enter the file name and location you want the PMML file to be written to in "Export model information to XML file" at the bottom of the "Save" box.
After the model is trained, a file will be created in the specified location containing a PMML representation of your linear regression model. A similar sequence of actions and results should work for Multinomial Logistic models.
Important things to notice about the PMML file:
1) The model is represented as a general regression PMML element;
2) For SPSS versions up to version 14, data transformations are not part of the PMML file. Therefore, you will need to add any data transformations manually. You can use the Transformation Generator tool to graphically design your transformations and then paste the resulting PMML code into your file.