If the name is already in use, the dev is obligated to use another, unique name.
If you use their name by simply adding characters to it, expect push-back during review...it could be viewed as an attempt to piggy-back on another app's popularity as a copy-cat, etc.
See the ASRGs:2.3.7 Choose a unique app name, assign keywords that accurately describe your app, and don’t try to pack any of your metadata with trademarked terms, popular app names, pricing information, or other irrelevant phrases just to game the system. App names must be limited to 30 characters and should not include prices, terms, or descriptions that are not the name of the app. App subtitles are a great way to provide additional context for your app; they must follow our standard metadata rules and should not include inappropriate content, reference other apps, or make unverifiable product claims. Apple may modify inappropriate keywords at any time or take other appropriate steps to prevent abuse.
If you think you have the rights to a name that is already in use, App Store Support says this about that:App Name Issues
If you believe that an app is preventing you from using the app name that you have rights to use, you can submit a claim at the following link. Where appropriate, Apple Legal will forward your contact information and claim to the developer of the blocking app so that they can contact you directly.
App Name Dispute
