No, the constraint of the last step parameter to be the negative sum of the rest is a necessary identification constraint for the model.
An easier way to interpret the rating scale or partial credit model you are estimating would be to look at the Thurstonian Thresholds (e.g., table 7 in the Show: `show ! tables = 1:2:3:4:7;`). In your example of it item scored 0, 1, 2, there will be 2 thresholds, labeled x.1 and x.2 (where x is your item numbers). Each threshold can be interpreted as the location on the latent trait at which the probability of achieving score >= 1 is > 0.5 (x.1) and score >=2 is 0.5 (x.2).
