It all depends on what your overall goals are and what your routine looks like. Doing an extra set of pullups a couple hours before you do them in the gym wont affect you much, if at all. For example, we have a pullup bar in our smoke pit. Sometimes when i go out ill sometimes pump a couple pullups out, while smoking ;-) Doesnt affect my back workout later that day. But why do you only work each body part once a week? Unless you are just doing maintenance work (which i have a feeling you arent since you are trying to add more stuff in), you should be doing each body part more than once a week. About 2 or 3 times...