If your data is based on a general observation class, you can determine the answer to this question by consulting the SDTM. Qualifiers for your general observation class, timing variables, and identifiers can generally be added to the dataset. There are some restrictions (e.g., you can't add --OCCUR to the AE domain) that can be found throughout various parts of the SDTMIG and have been gathered into the "Usage Restrictions" column of the tables in the SDTM v2.0.
Even if you're preparing a submission based on an earlier version of the SDTM, the SDTM v2.0 can serve as a resource to help you find restrictions that may apply in the earlier version and allow you to search your SDTMIG version to see whether it does.
Most SDTMIG domains include a "generally not used" assumption. For example, the last assumption for the Microscopic Findings (MI) domain says, "Any Identifiers, Timing variables, or Findings general observation class qualifiers may be added to the MI domain, but the following qualifiers would generally not be used: --POS, --MODIFY, --ORNRLO, --ORNRHI, --STNRLO, --STNRHI, --STNRC, --NRIND, --LEAD, --CSTATE, --BLFL, --FAST, --DRVFL, --LLOQ, --ULOQ." The fact that a variable is listed in a "generally not used" assumption does not prohibit its use; it simply reflects the experience of the SDS team, who did not think there was a use case for these variables in this domain. The last assumption of the ECG Test Results (EG) domain adds the sentence, "It is recommended that ‑‑LOINC not be used." Although this sentence is more strongly worded, it is still a recommendation; it does not prohibit the use of EGLOINC.