Switch to the absolute section, and set the section offset to expression, which must be an absolute expression. You might use this as follows:
.struct 0 field1: .struct field1 + 4 field2: .struct field2 + 4 field3:
This would define the symbol field1 to have the value 0, the symbol field2 to have the value 4, and the symbol field3 to have the value 8. Assembly would be left in the absolute section, and you would need to use a .section directive of some sort to change to some other section before further assembly.