if the design is originally created in Fireworks, you can use the Fireworks LIVE Filter > Inner Shadow.
Add a new vector object above your image, apply "Inner Shadow" and play with blending modes.
If you prefer Photoshop Inner Shadow, you have to modify it in Photoshop instead. Thats the only way.
Fireworks does not allow change of Blending Mode for Photoshop Filters.
Fireworks does not allow change of Blending Mode for Photoshop Filters.Why?
I'm assuming you're trying to make something like a button. If so please see below for a way to achieve a similar effect.
Step 1: Apply a stroke using fireworks default stroke feature. Align stroke to inside.
Step 2: Using fireworks live filters apply your first inner shadow as say a 35% opacity 0 pixel shadow shifted down 2 pixels set at 270º.
Step 3: Using fireworks live filters apply your second inner shadow as say a 100% opactiy 0 pixel shadow shifted down 1 pixel set a 270º. This shadow should be the same color as the stroke you created earlier.
This should yield the same effect as photoshop live effects. See the attached picture for side-by-side comparison. Left is using all fireworks live filters (except for the shadow) and right was an imported button that has all photoshop live effects.
Note* You can move fireworks filters up and down in terms of hierarchy just like layers. This is important to know incase you ever create your inner shadows out of order.
Hope this helps!