Modify filename on windows XP

[caglieris_gm]

I am a Windows XP user.
In order to manipilate multiplies window file, I need to inserte a suffix after filename (before .filetype).
For istance: in
For %%a in (*.jpg) do convert %%a -crop 1700x2700+0+0 .......
where *.jpg are 01.Jpg, 02.Jpg, exc and resulting files (......)
01_a.jpg, 02_a.jpg and so.
How can I do this?

[snibgo]

%%~na_a.jpg