Will the plugin recognize pre-existing meta fields?

No, but it is easy enough to write a small PHP script to convert a standard WP database to a Magic Fields 2 database. If you read some of my earlier posts in this forum you can probably understand enough to write this script. If not, I will be back here in a couple days – sorry no time tonight for a detailed explanation.

My wordpress blog is hosted by someone else and I do not have direct access to the database via phpAdmin or anything like that. Will such a php script still work?

I should have written “easy enough” as “easy enough for an experienced WordPress PHP programmer”. The script to do this is small (about 20 lines of PHP) but does require advanced knowledge of WordPress and PHP. From the nature of this question I surmise that you have limited knowledge of WordPress and PHP and the implementation of this script will probably be too difficult for your current skill level. (It is short but not really an easy script.) Please do not take offence at this. There was a point time when I too would have asked a question like this and at that point in time I would have been several weeks or months away from the skill level to do this. I keep forgetting that this forum is also for non coders and should have better qualified my answer.

My 2 cents:

Although I have selfish reasons for wanting you to use Magic Fields 2 – as I am the author of a Magic Fields 2 helper plugin – I feel it is only fair to warn you that Magic Fields 2 does not appear to be under active development – there have been only 2 maintenance releases (i.e., bug fixes) and no new features have been added since the initial release almost 2 years ago – and therefore would appear to have a limited future. Personally, it would not be my choice for starting a new project.