The new resisting arrest charge does not automatically add time to the existing 7-month sentence. The two matters are legally separate and get handled independently through their own processes.
The original 7-month sentence moves forward on its own timeline without being directly affected by the new charge. What the new charge does is create a second legal matter that will eventually come before a judge, and that is where the real risk to additional time exists.
On its own, a resisting arrest charge for a first-time offender with no prior history would likely result in probation rather than additional incarceration. Judges generally reserve jail time for more serious offenses when someone has no significant prior record.
The complication here is that this person already has a pending or active sentence when the new charge gets heard. Two separate legal problems arising in a short period tells the judge a different story than a single isolated incident. That pattern of behavior affects how the court views the resisting arrest charge and reduces the likelihood of a purely probationary outcome. Instead of probation, the judge may impose additional county jail time on top of the existing sentence, potentially adding several more months depending on the jurisdiction and the specific circumstances of the arrest.
The most important step right now is getting an attorney working on the new charge before it reaches a hearing. A lawyer can negotiate with the prosecution, argue for a disposition that does not result in additional incarceration, and present the judge with the most favorable possible framing of the circumstances. Without representation the outcome defaults to whatever the court decides, and that default is rarely the best available result.