Throwing my hat in the ring, my advice is to use time history for this analysis and use spectral analysis only as a measuring stick.
Spectral analysis considers maximum responses of modes only, independent of timing issues, summing them as best we can. Even with a single impulse, we have an approximation that is more accurately evaluated by time history analysis. Attempting to sum multiple impulse events based only on their maximum modal responses becomes less accurate.
We usually say 'use spectral analysis for slugs' because we have a poor approximation of the problem, therefore, why bother deriving an accurate solution? The answer is: "Because we have a timing issue."
Even if we can combine contributions of multiple DLF curves in CAESAR, the summation is independent of timing, which is what this problem is all about - Spectral analysis becomes limited in its validity.
My advice is to predict the force time profiles that are anticipated and perform a time history analysis.
The article Rich Ay (ray) refers to is very instructive, and is very helpful in terms of how to input the data; I recommend that you refer to it.
As far as input goes, it is helpful to manually plot the graph, then input the data corresponding to the plot.
In my opinion, the timing issue is about strain energy.
Using a simple loop as an example, if a slug hits one of two elbows in the far end of an expansion loop, and the loop moves, what happens when the slug hits the second elbow depends entirely on timing. If the energy from the first elbow impact is putting energy into the system, then the effect of the impact on the next elbow will be additive. If it the system is moving the other way when the impact hits, it will offset the load.

One issue to keep in mind is that CAESAR evaluates the force-time input differently for time-history than it does for spectral analysis input.
In time history, although the input prompts for force(lb) input, you should input a factor, such as 1.0 for full force, 0.5 for half. The Spectral analysis will give you the correct DLF curve whether you input the actual load or not, but the time history requires that you enter the force in either the force time input, or the force spectrum input. If you put it in both, they will be multiplied.
The article Rich mentioned will assist you in this also.
Hope this helps.