how to add a data file?

I have 2 accounts on my laptop running Vista Home Premium. One for me and one
for my wife. Data files were stored in different locations
C:\Users\<me>\...\Outlook.pst and C:\Users\<wife>\...\Outlook.pst. Well we
couldn't see inbox of each other while we are sharing the same email address.
I wanted to add her data file to my Personal Folders and my data file to her
respectively.
When I try to add the data file I've got the error:
"The file C:\Users\<wife>\...\Outlook .pst cannot be opened". I check that I
do have Full permissions for this file.
What I'm doing wrong?
Thanks in advance.
susja