In SDK you can create "External Web Service Integration" (.wsid) so that you could access external webservice in the business logic.
For this you need to do the following:
Upload .wsdl file and optionally .xsd files if you have schema in different .xsd files of the external webservice. This would create .wsid file, then you would activate the .wsid.
Suppose, If you have created .wsid uploading .wsdl having .xsd reference, and activation of this .wsid gives error "Recheck WSDL and upload again", In such case check your .wsdl file, it should not contain schema location (.xsd) refering to remote location.
Ex: your .wsdl has the following import tag for xsd:
download the abc.xsd from the remote location, change the schemaLocation in the .wsdl to schemaLocation="abc.xsd",
and then upload the .xsd into .wsid using the Add button mentioned in the below image.
You could add the .xsd file using the Add button highlighted below:
Note: The same holds good for .xsd file also, where you are refering to another remote .xsd withing the current .xsd file, In such case you need to follow the same solution, download the remote .xsd and upload it also changing the reference to local from remote.