missing variable

#1
Items include the scale of 0 (Does not apply or not used) to 3 (Used a great deal). I want to combine several items to make one scale. I assigned a missing variable to 9 because items already had 0. But when I measured mean of one scale (combination of items), it is out of range of 0-3. In this case, can i assign a missing variable to 0? If not, can i assign a missing variable to 0.1? Looking forward to hearing from you.
 
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