Importing certificates to be used in Outlook 2010 is possible only if they are not included in the contacts folder.
But Windows Mail and Outlook Express doesn’t export the certificates when exporting their address books so importing Microsoft Outlook Express address book with pubic key of recipients to outlook 2010 is a tricky procedure that involves exporting the certificates one by one as follow:

1.Open Windows Mail <select “Tools” < “Windows Contacts”. or
Open Outlook Express <select “Tools” < “Address Book”. Or
Open Windows Live Mail < select “Go” < choose “Contacts”.
2.Double click the name of the contact which his certificate needs to be exported.
3.On Windows Mail < Click the IDs tab.
On Outlook Express < Click the Digital IDs tab.
4.Select the requested certificate to export and click “Export”.
5.Save the certificate to a file. Outlook Express  and Windows Mail use the .cer
extension.
6.Open Outlook 2010 < open “Contacts” folder <open the contact item for the
person that owns the certificate that was imported on section 5.
7.Click “Certificates” < “Import” <select the file created in section 5 < click “Open”.
8.Save contact form.