I am currious why nearfields cant or shouldnt be used for mastering. Could they be used outside of the nearfield positioning?
Nearfields were originally proposed as a way to deal with large consoles which get in the way of stand-mounted loudspeakers. But as large consoles are disappearing, this justification goes away. Project studios often put nearfields on tables, which cause serious acoustical anomalies such as resonances and comb filtering. Nearfields have often been cited as helping to reduce acoustical problems of bad rooms, but all the other problems they introduce hardly justify their use.
One problem is that nearfield monitoring is like wearing big headphones! The stereo imaging is so wide that it discourages you from making a “big” master that will translate to home systems. The second problem is that the high frequency response of speakers that are to be used as nearfields has to be tailored for such close use, so they won’t bite your ear, so not just any speaker can be used as a nearfield. The third problem is that very few of the speakers designed as nearfields have adequate dynamics and low frequency extension (with some exceptions, I’ve seen engineers use Meyer HD-1s as nearfields, but these can sound overbright when used this close). The fourth problem is that nearfield monitoring exagerrates transients and affects your perception of the relationship of lead and solo versus rhythm. The fifth problem is that nearfield position exagerrates ambience, creating a higher ratio of direct to room sound. So nearfields are not particularly good for anything, either mixing or mastering!
Mixes and masters made on nearfields will have a great deal of trouble translating to other systems. I don’t recommend nearfield monitoring for any purpose except in remote truck control rooms with extremely limited space, where they are usually not used for mixing, but to verify that the recording (tracking) is going well.
To answer your question whether speakers designed to be used as nearfields can be used as mid- or farfield speakers, I doubt it. Most speakers which people are using as nearfields have so little headroom or extension that they will sound even worse when placed in the mid or far field! But there are some exceptions, and I find a pair of Genelec 8040s or 8240s make good midfields if not played too loudly.