How do I extend the output (high) time of my circuit (retriggerable)?
To use a 7555 here you have to deal with two problems, first you have a high going control signal and second how do you retrigger the 555.

The first is fairly easy you use a simple transistor inverter to invert the input signal.

The second takes a little more understanding of how the 555 is working. It charges up the timing capacitor until it reaches a threshold voltage and then turns off. You can retrigger it by discharging the capacitor.

Look at this circuit.

 

Q1 and R1 invert the incoming signal to make trigger active low Q2 discharges the capacitor while the input is high giving you the retrigger function. Note with this circuit your timing period is from the input signal going low.