The above sentence was an example sentence in a section of "Du's Handbook" that said 焉 (언) could "be used instead of 然 (연) as a suffix for an expressive adverb." In other words, the Handbook is saying that 焉 can be used similar to the adverb suffix "-ly" in English or the "히" in Korean.徐徐焉實狼其中“very slowly and gently, he placed the wolf in it [i.e. the bag]”
My question is why would an adverb suffix be necessary if the position of the adverb in front of a verb would normally be enough to clarify its function? Moreover, the double character 徐徐 (서서) is used exclusively as an adverb, so there would have been no mistaking it for anything else. Therefore, I suspect 焉 (언) was used here for another reason.
My Korean dictionary says that one of the meanings of 焉 (언) is "어찌" or "어떻게," which mean "how." In fact, that is the first meaning listed in my dictionary. In Korean, if you wanted to say "somehow," you would say "어떻게든," "어떻게 해서든," or "어찌 해서든," which is how I think 焉 was being used here. Therefore I would like to suggest the following translation:
"Somehow" is also an adverb, which would explain why it immediately precedes the verb 實 (실). That means the verb in this sentence has two adverbs, "slowly" and "somehow."徐徐焉實狼其中“Slowly (徐徐), [he] somehow (焉) placed (實) the wolf (狼) inside it (其中).”
Again, there may be times when the function of 焉 (언) can only be described as an adverb suffix, but I do not think this was one of those times.