Adding to the output of a DC/DC Boost Converter using the same power supply as the input?
Your 0.6V....36V converter is said to be buck-boost converter in the linked advertisement. 

Buck-boost is a well known principle where the same circuit can produce with acceptable efficiency and peak input current as well higher and lower output voltages than the input voltage. The circuit doesn't need a transformer, a single inductor is enough. It has advanced controller IC and the needed switching transistors and diodes for both buck and boost operating modes. The controller selects the needed mode automatically. 

Here's an example of the buck-boost idea. In this case it's used for broad input voltage range. https://www.analog.com/media/en/technical-documentation/data-sheets/8390fa.pdf

Boost converter actually adds in series with the input voltage extra DC which is generated as inductive pulses and stored to a capacitor. So, the 12V DC input is already used in series with inductively generated DC. It's no more availabe to be connected in series another time. That would be a short circuit if we assume your linked converter really operates as common boost coverter when it should increase the voltage. No datasheet = the already purchased circuit must be reverse engineered.

But you can make an inverting switch mode converter which generates -12V for you. Between its -12V output and the +36V output there's 48V. Unfortunately we do not know surely is the GND of the 36V converter at the output side same as the GND of the input side. If it's not something unwanted can happen. In addition the resulted 48V wouldn't be against GND, it would be +36V and -12V with GND not connected. 

Another possibility is to use your battery idea. Have a flyback converter which has fully isolated output side.

You wrote "No PWM, it must be continuous current" Unfortunately all dc to dc converters eat current pulses from the input supply voltage. They can be smoothed with a LC low-pass filter. Designing it isn't trivial. Actually many converters have a LC input filter, but its smoothing operation is designed only to prevent radio interference. You may need much more effective filtering.