Having searched the SAS support website
http://support.sas.com/ I find reference to p-values but not p-levels. Recommend visiting the SAS support site for DOC and technical papers. Also, a Google advanced search using the argument below also will generate some matches:
Glimmix p-values site:sas.com
I hope this info is helpful, as I admit to not having personal experience on your topic question.
Scott Barry
SBBWorks, Inc.
SUGI/SGF technical paper:
189-31: A Comparison of the MIXED Procedure and the GLIMMIX Procedure
http://www2.sas.com/proceedings/sugi31/189-31.pdf
SAS Documentation:
http://support.sas.com/documentation/