In the catalogue editor you could copy an existing valve to create a new valve, modify the size, dimensions etc to have the correct face to face dimensions and end conditions. However the ability to modify the valve operators are limited in the current release. After placing in the model you have an option to alter the operator shape and size using the property palette, again limited choices. Alternative is not use some AutoCAD solids to represent the operators.
You also have an option to place a custom component directly in the model and key in the dimensions at the time of placement.
Siva (Autodesk Singapore)