Is it possible to create my own LOINC code and include it in a resource/structure definition?
Only the organization responsible for defining a code system can add a new code to that code system. You can see the process for submitting proposals to LOINC here: http://loinc.org/submissions
Alternatively, you can define a code system of your own (with its own URL) and define additional codes there - and then use a value set containing a mixture of LOINC and non-LOINC codes.
To the best of my knowledge, there are no places in the base FHIR spec where codes other than LOINC are prohibited, though the vital signs profile does require the use of specific LOINC codes for specific concepts.