Converting negative voltage to positive
Between the +5V and the -12V outputs, you have a total of 17V, definitely not what you need. Connecting the two together will damage the power supply. Not good either.

The proper way to get +7V is to use a buck regulator to regulate the +12V down to +7V with good efficiency. This is usually easier than to convert from a negative voltage.

The Alibaba web site is choke-full of buck converters of various current ratings that would certainly work just fine.

If your current demand is low (a fraction of an ampere), a linear regulator may work but they tend to get hot unless they are heat sunk. A Buck regulator is easier to implement.

You may need to read in general about buck regulators to understand how they work and apply them effectively.