If the company has a developer Enterprise license, then you can take your source code and build and sign using certificates created from their account. This version of the app can then be distributed within the company. Note that if this company does not currently have an Enterprise developer account, they will have to sign up for one (requires a D-U-N-S number) and will need to incur the annual cost of $299.
There are a number of things that you need to deal with. You will need to deal with ownership of your code, ongoing support and recompiling the app for them annually when the certificate expires, etc. since you are not an employee of this company.
This question will probably get closed for being off-topic, but hopefully gives you something to think about.