I would suggest that you write a sql script to do this easily, but it will require that you know or can get the concept ids. You can write a select statement to add all the concepts with a class = diagnoses, and then remove all the non-respiratory diagnoses, but I am guessing you have many diagnoses.
First create the concept set (use the ui since this is easy). Class is ConvSet; Datatype is N/A, Checkbox selected for set. For this example, the concept_id for this concept (1234) (concept_set), the creator of '1' is the admin user, and the respiratory diagnoses are concept_id 101, 102, etc :
insert into concept_set
(102, 1234, 1, uuid()),