Each of the outer order rituals consist of various threads and structures interwoven into a more or less coherent whole. You can make use of some of these structures without necessarily including the other aspects of the ritual.
For instance, the 2nd part of the Zelator ritual refers to the outer court of the Tabernacle in the Wilderness, associating key elements of that structure with the Hebrew letters in their 3 categories of single, double and mother letters. It wouldn't be difficult to construct a ritual that elaborates on that structure, perhaps involving a brief meditation on each of the 22 letters with their colors in the king scale and primary associations.
Years ago I put together an invocation of fire based on the Path of Shin section of the Practicus ritual. The invocation consisted of separate segments for solar, astral and volcanic fire. The temple was arranged according to the fire triangle (with stations at 3 points, 2 basal in the west and the apex in the east), and included the litanies from the Chaldean Oracles which are spoken by the officers in the Practicus ritual. Further elaborations were added and the ritual turned out to be quite effective (I've performed it alone and with a small group).
Another thing to look at is the Z2 formula. I personally think that the Z2 formula (found in the small black GD tome published by Regardie) is a bit of a mess. It's overcomplicated, emphasizes the minutia and overlooks key elements of the ritual. I've rewritten it from the ground up several times (based on years of experience with the ritual in practice). I don't think my version is the be-all-end-all (though it works great for me), but the practice of rewriting it and creating rituals based upon it has been quite beneficial.
The same thing can be done for all of the Outer Order rituals - each of them have their own particular structure and dynamic (in fact, this is true for each part of each ritual - for example, there are four main rituals contained within the Philosophus ritual: Qoph, Tzaddi, Peh and Netzach. Each part fits within the whole in a particular way.) It would be quite easy, for example, to build a ritual based in the four stations of the sun (or, alternately, the phases of the moon) using the general structure found in the Tau section of the Theoricus ritual.
Anyway, the GD rituals are merely one among many ways of approaching these things. But, if you specifically want to make use of these rituals, it can certainly be done in a variety ways. You might argue that there are better ways to approach the western mysteries and you might be right. One thing I will add is that experiencing the rituals in full, both as a candidate and in various officer roles, is the best way to understand them. There is a lot going on in these rituals that doesn't come across in any way on paper.