We can insert new elements into the lists, but this is not applicable to arrays.
First, I suspect that "the" should not be included before "lists". If you are speaking about a specific set of lists (that you described previously in the paper or discussion), then including "the" could be appropriate. However, in this case, I suspect you are speaking of "lists" as a generic object and consequently, "the" should be omitted.
We can insert new elements into lists, but this is not applicable to arrays.
I believe this sentence is valid now. It definitely conveys the correct information and is easily understood but it doesn't sound 100% proper to my ears. If I heard it in regular conversation, I would assume that the speaker was not a native American English speaker, had not formulated his thoughts properly prior to speaking, or that he was trying to sound more technical than he was comfortably able.
I'm not sure exactly why it sounds slightly wrong but I'll try to explain by looking at two versions that I think are better.
(a little better) We can insert new elements into lists, but this does not apply to arrays."
(best) New elements can be inserted into lists but this in not true for arrays.
I'm pretty sure it has to do with how the word "applicable" is being used. The word "applicable" carries with it a strong implication that two sets of circumstances are being compared. What you are doing though is comparing two ideas: "insertion into arrays" and "insertion into lists". Usage of the word "applicable" also carries with it the connotation that if circumstances changed or had been different, the applicability might change as well. It seems to me that the strength of the above implication is not as strong with the verb "to apply" as it is with the noun "applicable". I really have no idea why this is and maybe some would disagree.
To try to illustrate my point. if you omitted the "to arrays" part from your sentence and asked someone to finish it, almost everyone would finish it with a variation of the following:
We can insert new elements into the lists, but this is not applicable in these circumstances.
As an aside, another way to phrase your sentence and probably the best is to use the verb "to support".
Lists support the insertion operation and arrays do not.
I cannot come up with a reasonable way to use "as if" or "as though" in this circumstance. Both of those constructions imply that two things are equivalent in a sense and cannot be used to accentuate differences.
The boy flew as though he were a bird.