For the purposes of Article 40 of Directive 2014/59/EU (BRRD) a bridge institution can be a legal person which already exists, but the requirement of Article 40(2)(b) that a bridge institution is created for the purpose of holding shares or other instruments of ownership, or assets, rights or liabilities of one or more institutions under resolution should be complied with (i.e., it cannot be used for other purposes). 

As regards using a legal person that is being held available for this purpose by the institution concerned, such a possibility is not the BRRD, as long as the bridge institution complies with provisions of Articles 40 and 41, such as the control of the institution by the resolution authority.

Disclaimer:

This question goes beyond matters of consistent and effective application of the regulatory framework. A Directorate General of the Commission (Directorate General Financial Stability, Financial Services and Capital Markets Union) has prepared the answer, albeit that only the Court of Justice of the European Union can provide definitive interpretations of EU legislation. This is an unofficial opinion of that Directorate General, which the European Banking Authority publishes on its behalf. The answers are not binding on the European Commission as an institution. You should be aware that the European Commission could adopt a position different from the one expressed in such Q&As, for instance in infringement proceedings or after a detailed examination of a specific case or on the basis of any new legal or factual elements that may have been brought to its attention.