The name and email address corresponding to the sending DocuSign user account will always be shown on/for sent envelopes. That being said, there's an easy way to achieve what you describe:
Create another "user" (i.e., membership) within the same DocuSign account -- this will represent the "system sender" user account -- i.e., the user account that you'll specify when sending envelopes with the API.
When creating the new "user", set name and email address to the values that you want to see/use for envelopes you send via the API.
Make sure to grant the new "user" the appropriate API-related settings (under the DocuSign API heading when viewing User Permissions).
Use the "system sender user" credentials to send envelopes via the API.
Keep in mind that when using this "system sender" user to send envelopes via the API, that user will be considered the sender of each envelope sent using that account's credentials -- which means that only that user's credentials can be used via the DocuSign web console to access the DocuSign envelope for viewing/correcting/etc. Other members of that same DocuSign account won't be able to access the envelope via the DocuSign web console unless you explicitly grant 'folder sharing' rights between users.