I’ve heard of "imputing income" when it comes to child support. Does it also apply to spousal support?

Yes, one can impute income when it comes to spousal support. "Imputing income" arises when one’s earnings have diminished substantially, while one’s "earning capacity" remains unchanged.

For example: if someone quit his job as a stockbroker and worked in a coffee shop, his or her earnings would diminish substantially. A judge could use the ex-stockbroker’s earning capacity to determine the support award, if it appeared that he or she deliberately quit his or her job and lowered his or her income. Keep in mind that this lowering of income does not have to be for the specific purpose of avoiding support.