I don't know if there is a gas station on the moon!
What you can do is to save your scenario (ctrl S), leave the sim , open your saved scenario with a text editor and change the values of the line PRPLEVEL, in the chapter "BEGIN SHIPS"
You will see different ones numbered from 0 to 4. I don't know exactly which one is related to the SPS fuel, but you can do several attempts; the others are for the RCS.
You will see for example: "0:0.154" that means you have 15,4% fuel left in the tank number 0. Change for "0:1.000" and you will have a plenty tank number 0.
TLI and trajectory to the moon requires great work and training.
For my own I use IMFD in offplan intercept mode to do that. But you must be very accurate during the computation. The best way I found to reached the moon while in Apollo 8 for example, and this mission is one of the most difficult to do because the planes are largely non aligned is to match the timeline as a prime goal. I mean that you may want to set the historical date and time both for TLI and arrival time behindf the moon. In this way you will be very close of the real trajectory which of course is valid with the CSM SPS fuel capability.
During the TLI computation, don't bother with the resulting equator inclination around the moon. If you depart and arrive on time you MUST be on the right trajectory and with the proper inclination. You may want to adjust the inclination during the midcourse correction; for information the Apollo 8 historical flight path was 12° inclinated related to moon equator, which is, in IMFD terms, 168° because it's a retrograde course: that means that you'r orbiting the moon from left side to right side as seen from the earth.
With IMFD the most hard to perform is to properly adjust the moon Pe to around 111km (60Nm) because the there is no IMFD program accurate enough to do that. It requires a sort of intuition or feeling or what you want. The best is to try different methods during training.
Remember too that the shape of the orbit is less important than the period. I mean that, for example, if your Pe is too high you simply may decrease your Ap to get back to the same orbital period. According to the Keplerian law (the first one no?) if the semi major axis of to orbits are equal the periods are equals too.
In this way, if you match the real orbital period you can complete the mission very close to the historical one.
I hope that this could help you a bit.
Have nice virtual spaceflights.